The long-term goal of this project is to determine the role of the Na ion-K ion ATPase in active chloride transporting epithelial. Our approach to the problem is the localization and counting of the receptors of drugs which inhibit salt transport in these epithelia. These drugs include cardioglycosides, for example, ouabain and the so-called high ceiling diuretics such a furosemide. The technique of high resolution quantitative autoradiography is used to localize these 3H-labeled drugs. Knowledge of the membrane location and density should yield a better understanding of the membrane mechanisms which actively transfer ions across epithelia such as the thick ascending limb and cornea.